There are numerous methods for supporting the roof of an under-ground mine. One such method is the cross-bar method wherein beams fabricated of wood, steel, or another material are placed against a mine roof. Each end of each beam is supported by a post made from any of the materials used in making the beams or, alternatively, from concrete. The crossbar method has the disadvantage that the posts can be accidentally knocked out by moving machinery, thus endangering the miners. To protect miners in such situations, cable or steel straps are bolted into the roof in order to support the beam should a post be knocked out. The beams can also be drilled and bolted directly to the roof. Installing crossbars is a slow and labor-intensive process, the materials are expensive, and installation can be hazardous. Moreover, wood is not a permanent material even if it is treated.
In another method, continuous bolt trusses are fabricated from angled roof bolts anchored into the roof by mechanical devices or adhesive resins. The bolts are connected by means of one or more tie-rods and a turnbuckle. Tightening of the turnbuckle can produce compressive forces in the roof rock which increases the strength of the rock. However, because the turnbuckle length or take-up is limited, the roof bolt holes must be precisely located or, otherwise, various lengths of tie-rods must be available to be connected to the roof bolts and turnbuckle in order to allow the truss system to be tensioned. Further, the threads cut or rolled into the ends of the roof bolts and tie-rods act as stress concentration points and also reduce the effective area of the bolt/tie-rod, thus reducing the effective ultimate strength of the system. Fine machine threads are subject to damage, rust, and corrosion. Still further, assembly of the continuous bolt multi-segment tie-rod truss system is time-consuming.
In a third method, multiple angled bolt trusses are fabricated by securing an end of each of two bolts at angles in the roof of the mine and by passing the other ends of the two bolts through plates or brackets such that each bolt is tensioned separately. Tie-rods, in two to five sections, are connected to the plates or brackets using turnbuckles or tensioning bolts and couplers such that the turnbuckles or tensioning bolts can tension the tie-rods. Since the tie-rods and bolts are tensioned separately, the compressive forces on the roof rock may be unequal. This may result in one bolt being overloaded close to failure while the tie-rod and opposite bolt have little or no stress. The roof bolt holes must be located at precise distances to allow tensioning within the limited range of a turnbuckle or tensioning bolt or else several sections of various lengths of tie-rods must be available to achieve the proper tie-rod length. This method suffers from the same drawbacks as the previous method described above.
In a fourth method, cable slings of lengths of wire rope are used to support the mine roof. The wire rope is attached to a split tube and the latter is driven up into a grout-filled bore hole by a split tube driver. However, variations in bore hole diameter due to drilling and/or rock movement hinder the passage of the split tube such that there is little control of the tension on the wire rope. After installation, some cables have no tension and must be blocked with wood to the roof and tightened with wedges. Also, the impact driving of the split tubes is slow and very noisy, and requires three operators to install a cable sling. Furthermore, impact driving of the split tubes can disturb the roof and ribs and may dislodge material thus endangering miners.